The term Simulacra was first popularized by Philip K. Dick in his 1964 book 'The Simulacra'.
It can loosely be used to mean a copy without an original or a pre-existing simulation.
An example of this is an android which is a copy or simulation of a human being but which can exist (can be created) without an exact same human being existing from which it is copied.
The Simulacra (the novel) describes a post nuclear war future in which a charismatic First Lady, Nicole Thibodeaux, has taken over the running of the USA by means of a succession of android husbands who are elected to the office of President. Her television appearences become the sole source of entertainment (or is it religion?) for the population who worship her beauty - though few notice that her physical appearence has not changed in over 50 years.
The notion of simulation and the problem of the copy being more real than the real comes up again in Dick's most famous novel - Do Androids Dream of Electric Sheep? - in which he explores the fine line between reality and a mere representation thereof.
Finally, the notion of simulacra was brought to philosophy by Jean Baudrillard in his 1981 book Simulacra and Simulation.
says re simulacra: Not coined by Dick; popularized perhaps. Simulacrum is a Latin word for image or semblance.